METABOLISM. Y5 



Associated with the peripheral ends of the nerves of touch 

 are certain small bodies — corpuscula tactics — each of which, 

 when disturbed by something in contact with the skin, 

 presses on the adjacent fibre more strongly than soft tissue 

 would do, and thus multiplies the force producing sensation. 

 While serving the further purpose of touching at a distance, 

 the vibrissce or whiskers of a feline animal achieve a like end 

 in a more effectual way. The external portion of each 

 bristle acts as the long arm of a lever, and the internal por- 

 tion as the short arm. The result is that a slight touch at 

 the outer end of the bristle produces a considerable pressure 

 of the inner end on the nerve-terminal : so intensifying the 

 impression. In the hearing organs of various inferior types 

 of animals, the otolites in contact with the auditory nerves, 

 when they are struck by sound-waves, give to the nerves 

 much stronger impressions than these would have were they 

 simply immersed in loose tissue; and in the ears of de- 

 veloped creatures there exist more elaborate appliances for 

 augmenting the effects of aerial vibrations. From this mul- 

 tiplication of molar actions let us pass to the multiplication 

 of molecular actions. The retina is made up of minute rods 

 and cones, so packed together side by side that they can be 

 separately affected by the separate parts of the images of 

 objects. As each of them is but TTj-.l-oirth of an inch in 

 diameter, the ethereal undulations falling upon it can pro- 

 duce an amount of change almost infinitesimal — an amount 

 probably incapable of exciting a nerve-centre, or indeed of 

 overcoming the molecular inertia of the nerve leading to it. 

 But in close proximity are layers of granules into which the 

 rods and cones send fibres, and beyond these, about -rio^th 

 of an inch from the retinal layer, lie ganglion-cells, in 

 each of which a minute disturbance may readily evolve a 

 larger disturbance; so that by multiplication, single or per- 

 haps double, there is produced a force sufficient to excite 

 the fibre connected with the centre of vision. Such, at least, 

 judging from the requirement and the structure, seems to me 



